This invention relates to the field of inboard power motor boats and more particularly to mechanisms to provide steering and trimming control of those boats. One of the more prevelant steering drive mechanisms used on present day inboard speed motor boats or pleasure boats is the inboard/outboard drive. This mechanism is comprised of a strut which is connected to the transmission drive of an engine through the use of a series of mechanical linkages, including conventional universal joints, two sets of 90.degree. bevel gears, two to three shafts, two to three bearings and many other smaller parts. These inboard/outboard arrangements are designed to combine the advantages of utilizing a large powered engine set within the boat hull with the advantages of maneuverability in an outboard motor boat steering mechanism which provides the ability to maneuver the propeller itself, giving better steering response.
The primary disadvantage to those conventional inboard/outboard steering drives is the loss of power as a result of the use of the rather complicated connecting mechanism between the power transmission of the engine and the steering strut on which the propeller is mounted. The requirement to have the power transmission shaft make two 90.degree. turns through two sets of bevel gears greatly contributes to the power loss which can be as much as 30% of potential power output from the engine. This can be of critical importance to inboard motor speed boats with regard to performance characteristics and also is of serious concern to fuel economy in pleasure boating.
Another reason for the use of an inboard/outboard stern drive mechanism is to place the engine to the stern of the boat in order to allow more space throughout the rest of the boat for other use. However, locating the engine at the stern of the boat presents a problem in the placement of the propeller drive shaft. Normally, when the engine is placed amid the boat, the extension of the propeller shaft to the stern of the boat provides a desirable low angle in the shaft. The low or small angle of the propeller drive shaft is necessary in order to produce the proper power thrust line for the boat for efficient movement through the water. However, the placement of the engine in the midsection of the boat is not conducive to pleasure boating, since the large power engine can be quite noisy and is situated in the middle of the living space. Consequently, various inboard/outboard drives or steering mechanisms were designed to enable the placement of the engine to the stern of the boat.
Some of the prior art inboard motor boats have utilized a pair of conventional universal joints to compensate for a higher angle for the propeller drive shaft. The problem with conventional U-joint systems is that the rotational velocity is not transmitted uniformly unless the transmission shaft and the propeller shaft are at zero offset. For any angular offset the output is a sinusoidal function with a complete acceleration and deacceleration for each turn of the shaft. The magnitude of these is determined by the angularity of the offset. Multiple installation of U-joints can compensate for parallel offsets between the power transmission shaft and the propeller shaft, but these cannot compensate for any angular offset. This becomes exceedingly important when considering the rotation of a system at higher velocity which spins at 7000 rpm or roughly a hundred times each second and the system has to bleed energy to accelerate and decelerate the mass involved. With the rotational inertias present, the system becomes non-rigid and elastic deformation of mechanical parts takes place. This then introduces the possibility of fatigue failure and in particular introduces another mechanical forcing function into a system which already has a similar frequency due to successive immersion of partially submerged propeller blades.